Temperature compensation of microwave cavity

ABSTRACT

A microwave cavity resonator is disclosed having thermal compensation means to substantially reduce the effects on operating frequency as a result of temperature variations. Cylindrical hollow cavity resonators defined by metals of a predetermined thermal expansion coefficient are provided with reentrant gap defining structures with such structures fabricated from or having associated therewith a compensating material of a lower thermal coefficient of expansion characteristic. The temperature compensating members introduce with respect to the frequency determining components of the resonator a substantially positive thermal coefficient of frequency to cancel and measurably offset the inherently negative thermal coefficient frequency variations due to the metallic composition of the cavity walls. Gridless as well as gridded cavity resonator structures comprise applicable embodiments as well as fixed frequency or tunable microwave devices.

United States Patent [72} Inventor Roger G. Carignan 3,297,905 1/1967 Fiedor et a1 333/83X Wilmington, Mass. 3,466,495 9/1969 Ward 333/83X 3; 2 1969 Primary ExaminerHerman Karl Saalbach l la 6 3 Assistant Examiner-Wm. H. Punter Assignee Raytheon Company I AnQorrggg-Harold A. Murphy, Joseph D. Pannone and Edgar Lexington, Mass.

[54] TEMPERATURE COMPENSATION OF \BSTRACT: A microwave cavity resonator is disclosed having thermal compensation means to substantially reduce the MICROWAVE CAVITY ff ts f l sclaimssvmawing Figs sageizcltiorosn opera mg requency as a resut 0 temperature U.S. Cylindrical hollow cavity resonators defined metals of a 315/552 predetermined thermal expansion coefficient are provided [51] lllLCl H01!) 7/06, with feentrant gap defining tructures with such structures H0113 H30, 23/70 fabricated from or having associated therewith a compensat- [50] Field Of Search 333/83 (T), ing material of a lower thermal coefficient of expansion 826T), 83,3; characteristic. The temperature compensating members introduce with respect to the frequency determining com- [56] References cued ponents of the resonator a substantially positive thermal coef- UNlTED STATES PATENTS ficient of frequency to cancel and measurably offset the in- 2,606,302 8/1952 Learned 333 83(T)X herently negative thermal coefficient frequency variations due 2,790,151 4/ 1957 Riblet 333/83 to the metallic composition of the cavity walls. Gridless as well 2,880,357 3/1959 Snow et a1. 333/83T(X) as gridded cavity resonator structures comprise applicable 3,034,078 5/ 1962 McCoubrey. 333/83(T) embodiments as well as fixed frequency or tunable microwave 3,292,239 12/1966 Sadler 333/83X devices.

50 48 2O 52 l y 7 2a 1 i 7 44 7 f y 34 -40 I 4 38* x i Q I 3 R Patented Apx 'il 6, 1971 s Sheets-Sheet 1 TEMPERATURE FIG: 2

INVENTOR 06E? 6. CAR/63M FIG. 4

TEMPERATURE COMPENSATION OF MICROWAVE CAVITY BAC KGROUN D OF THE INVENTION In microwave frequency devices embodying cavity resonators, the reduction of frequency sensitivity with respect to temperature is highly desirable. Many temperature compensating materials and techniques have heretofore been employed for thermal compensation of tuning members in cavity resonators. In addition, materials having low thermal coefficient of expansion characteristics such as Invar have been employed to fabricate the cavity resonator wall structure or to constrain movement of such structure through temperature variations. The introduction of foreign materials which includes those having low thermal coefficient of expansion as well as brazing materials possibly associated therewith has seriously lowered the electrical conductivity of the cavity resonators. It is necessary, therefore, that such materials be electroplated with high electrical conductivity metals such as copper which is both time consuming and expensive by reason of the special techniques required to control dimensions, plating thickness as well as uniformity.

Constraining means of a low thermal coefficient of expansion material in communication with cavity resonator walls have limited use and would be of little consequence in reentrant-type cavity resonators where the internal gap defining structures are generally provided.

At higher frequencies, particularly K -band and higher, where, for example, at 30 GHz. a cavity diameter of only 0.175 inch is realized, frequency sensitivity with temperature becomes an even more important parameter to be taken into consideration. The provision of cavity resonators having substantially reduced frequency sensitivity to temperature variations is therefore desirable for all such microwave devices as klystron oscillators and amplifiers of the plural as well as singular cavity configuration as well as any device incorporating reentrant cavity resonators.

SUMMARY OF THE INVENTION A cavity resonator for microwave devices of the reentrant type having central capacitive gap defining means is thermally compensated by improvement of the structure associated with such gap defining means. The invention encompasses the introduction of a material and dimensions to increase positive thermal coefficient of frequency factor which substantially cancels or nullifies the inherent negative thermal coefficient of frequency factor of the uncompensated cavity. Preferably, a material having a thermal coefi'rcient of expansion sufficiently lower than the cavity wall material is provided to yield overall compensating effects as near to the idealized conditions of a zero thermal coefficient of frequency value. The gap defining means which are commonly of a conical or cylindrical configuration are fabricated of the material having the lower thermal coefficient of expansion or a member of such material is embedded in the cavity resonator wall structure closely adjacent the conical gap members. The net effect of the improvement is to constrain movement of the capacitive frequency determining members over the temperature range. With movement of the remaining cavity resonator walls and even the opposing conical member if only one such member is thermally compensated the capacitive gap will become even wider with temperature than a gap without thermal compensation. The accelerated gap widening tends to increase the frequency at a faster rate as the temperature increases than an uncompirsated structure. The dimensions and total quantity of material required to yield the desired thennal compensation may first be calculated by an analytical expression and then adjusted empirically in an operative device. The amount of so-called foreign or low electrical conductivity materials within the cavity resonator is reduced to a minimum or eliminated altogether to substantially reduce the prior art disadvantages in the electrical properties of such resonators.

BRIEF DESCRIPTION OF THE DRAWINGS The invention as well as the details of the construction of several embodiments will be readily understood after consideration of the following detailed description in reference to the accompanying drawings, in which:

FIG. I is a diagrammatic representation of a reentrant cavity resonator;

FIG. 2 is an illustrative graph of the frequency variations relative to temperature variations of prior art uncompensated cavity resonator structures;

FIG. 3 is a partial cross-sectional view of an illustrative embodiment of the invention;

FIG. 4 is a top view of the embodiment shown in FIG. 3;

FIG. 5 is a diagrammatic representation view of an embodiment of the invention illustrating the cavity dimension variations with temperature;

FIG. 6 is a graph of relative frequency versus temperature for a compensated and uncompensated cavity resonator;

FIG. 7 is a cross-sectional view of an alternative embodiment of the invention;

FIG. 8 is a cross-sectional view of another illustrative em bodiment; and

FIG. 9 is a partial cross-sectional view of still another alternative embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a cylindrical microwave cavity resonator of the reentrant type 2 having oppositely disposed conical members 4 and 6 defining therebetween a capacitive gap 8. Such cavity resonators are commonly fabricated by machining a block of a high electrically conductive metal such as copper. Inner cavity walls 10 are defined and a central passageway 12 through the conical members is traversed by an axial electron beam from any conventional well-known electron gun assembly. Any number of such cavity resonators can be employed in amplifiers as well as oscillators and the electron beam is collected adjacent the last or output cavity resonator in such devices.

Resonant frequency of such cavity resonators is influenced by the gap providing the capacitive component and the cavity volume providing the inductive component. It is understood for the purposes of the present description that appropriate electric fields are applied within the cavity resonator and the radio frequency currents flow along the inner walls as well as conical gap members centrally disposed therein. Further, it is assumed that the axial electron beam may be focused by mag netic members disposed parallel to the direction of the propagation of the said beam. In most devices such as for example klystrons, having one or two cavity resonators, electrostatic field focusing of the beam is sufficient and magnetic members are unnecessary.

The resonant frequency of the cavity resonator is determined by the equation v volume with temperature thereby increasing the inductance.

Referring to FIG. 2, an illustrative graph of such fluctuations with the frequency designated along the vertical coordinate and temperature along the horizontal coordinate results in a line 14. It is noted that as the cavity volume increases with increased temperature the frequency decreases and this characteristic is depicted in the negative slope of the line 14. We refer therefore to an uncompensated cavity resonator as having an inherent negative temperature coefficient of frequency. In reentrant cavity resonators the conical members do provide some compensating effect in that as the gap becomes larger the capacitance decreases and the resonant frequency tends to increase. This effect results in a positive temperature coefficient of frequency as indicated at line 16 in FIG. 2. The overall effect indicated by dashed line 15, however, will generally rest on the negative side since the cavity volume effect is substantially larger.

in accordance with the invention a reentrant-type cavity resonator is thermally compensated to yield a result as close to zero temperature coefficient of frequency as possible by increasing the positive effect of the temperature coefficient of frequency. This is accomplished through appropriate selection of lower expansion materials such as the nickel-iron alloys or low expansion elemental metals such as molybdenum, and their placement to provide an even greater positive effect of the capacitive gap members with respect to frequency as the temperature increases. in F165. 3 and 4 an embodiment of the invention is illustrated. in this view a portion of a multicavity structure has been shown fabricated from a block of metal, commonly copper. For the sake of clarity in the understanding of the invention since it relates to thermal compensation of cavity resonators appurtenant structure such as the electron beam gun assembly, focusing and accelerating electrodes, collector electrode, output and/r input coupling means, as well as magnetic focusing means have not been shown.

The cavity resonators 20 and 22 are fabricated from the highly conductive metal in two sections 24 and 26. Each of such sections is provided with a reentrant conical electrode wall portion 28 and 30 and a central axial passageway 32. An enlargement of the axial passageway 32 is provided adjacent the opposing end of each section defining shoulders 34 and 36. A thermal compensating body member 38 of a material having a substantially lower thermal coefiicient of expansion such as any of the nickel-iron alloys or other suitable materials is positioned abutting and finnly attached to the shoulders 34 and 36 so as to be closely adjacent and control movement of the conical members 28 and 30. The compensating member is brazed or otherwise embedded within sections 24 and 26. The metal lnvar has been used with excellent results and for applications requiring magnetic focusing molybdenum can be used to the magnetic properties of lnvar.

in the illustrative embodiment dual back-to-back cavity resonator structures have been disclosed and each of said conical electrode members will have an oppositely disposed member associated with it to define a capacitive gap as shown in FIG. I. The remainder of the structure shown in FIG. 3 as well as H0. 4 includes a step-shaped portion 40 to define the conventional output transformer section 42 and output waveguide means 44. The overall cavity configuration defined by abutting members 24 and 26 may be of a square configuration. An iris opening 48 provides for feeding energy from output cavity 20 to input cavity 22 through an enclosed feedback waveguide path 50. An output iris opening 52 in cavity section 24 provides for external coupling of the microwave energy to output waveguide 44.

The resultant frequency changes with temperature in the embodiment of the invention will now be explained with reference being directed to FIG. 5. A copper cavity block member 54 defines a reentrant cavity 56 with conical electrode members 58 and60 providing therebetween a gap 62. A thennal compensating cylindrical body member 64 is embedded within the block member closely adjacent to the wall structure-defining conical member 60 and is secured by metallurgical techniques to all contacted walls of the block member. An axial passageway 65 provides for passage of the electron beam. The peripheral surfaces of the cavity block member as well as reentrant cavity and gap members are indicated by solid lines 67 for dimensions at the lower temperature range. The dotted lines 68 indicate the high temperature dimensions. A reference plane 66 is centered on the length of the compensating member 64 and the dimension 1 is taken as half of its total length. The aforesaid lines 68 of the respective members are an approximation for illustrative purposes to assist in an explanation of the invention. The conical gap electrode member 60 of the material, specifically copper, having the higher thermal expansion characteristic, is inhibited or restricted in its movement relative to reference plane 66 due to the presence of the material in cylinder 64 having the substantially lower thermal coefficient of expansion. The gap 62 therefore instead of widening with temperature at a more uniform rate with both cones expanding is forced to expand at an accelerated rate because conical electrode member 60 is restricted. The wider gap increases frequency at an accelerated rate and the compensating effect of the positive thermal coefficient of frequency characteristic is provided.

The dual cavity resonator embodiment shown in FIGS. 3 and 4 will have both cavities thermally compensated by the same compensating member 38 which will be of sufficient length to be closely adjacent both conical electrodes 28 and 30. Additionally, the opposing conical gap-defining members may be thermally compensated in either the single or multicavity configurations. The amounts of lower expansion material required for the net compensation effect may be readily provided by other additional structures such as fabricating two back-to-back gap conical members as well as the intermediate supporting structure entirely from the material having the lower expansion rate. Other modifications will also be evident as will be hereinafter described. In all such embodiments the interior cavity wall surfaces will still be fabricated from the desired high electrically conductive material and additional electroplating of such wall surfaces may be eliminated. The important parameter then to be considered is the acceleration of the positive coefficient of frequency characteristic.

A technique useful in determining the appropriate compensation required for selected cavity resonators will now be described using an analytical expression. It is assumed that all the theoretical cavity dimensions have been calculated and a cold test model yielding a predetermined resonant frequency and the desired electrical parameters has resulted. Such a cavity resonator in an exemplary embodiment having a resonant frequency of 30.0 GHz. had the following measurements: overall cavity diameter 0.175 inch; height of the cavity at the outermost walls 0.092 inch; gap dimension 0.016 inch and electron beam passageway 0.040 inch. A measurement of the temperature coefficient of frequency in the uncompensated state indicated a value of 570 kHz. per degree centigrade and is indicated by line 70 in FIG. 6.

To arrive at the calculation of the temperature coefficient of frequency the following expression may be utilized:

C temperature coefficient of compensated cavity;

C0 temperature coefficient of the uncompensated cavity;

a gap tuning rate;

I length of compensating material;

k, thermal coefficient of expansion of uncompensated cavity material; and

k2 thermal coefficient of expansion of compensating materials to be used.

it will be noted that the temperature coefficient of the uncompensated cavity has a negative value while the term al(k ,k is positive when k is greater than k and may therefore be similar to the compensating effect which is attributed to the introduction of the gap-defining electrode members within the cavity. A zero temperature coefficient will result when the value of al(k k is substantially equal to the uncompensated temperature coefficient. in the utilization of the foregoing expression the general procedure is to measure a cold test cavity or operative tube which has no compensation and determine the temperature coefficient C0. The gap tuning rate a may also be easily measured by frequency readings at various gap spacings. Once these two quantities have been determined the amount of compensating material as well as the type of material may be selected to achieve the desired positive temperature coefiicient to arrive as close to a zero value as possible. The final adjustments can in most instances be determined empirically.

In the exemplary embodiment having the 570 kHz. per degree centigrade uncompensated temperature coefficient the measured gap tuning rate was 390 MHz. per degree centigrade. Since the spacing of the gap defining members in the dual cavity configuration is substantially fixed by the electron beam requirements an overall dimension of 0.280 inch was the maximum length permissible in the area between the cone members. Since by definition I is taken as half the total length of the compensating element, a maximum value of 0.140 inch is therefore utilized in the solution of the equation for a first approximation This results in a required value for k of 6.2l l0 inch per inch per degree centigrade. A material close to this value is available under the trade name Kovar having an expansion rate of 5.95Xl0 inch per inch per degree centigrade. Experimentation in a cold test model resulted in a Kovar sleeve having a length dimension of 0.123 inch and diameter of 0.l00 inch. The calculated temperature coefficient for a cold test cavity with this compensating member was 3l0 kHz. per degree centigrade. As shown by line 72 of FIG. 6 this value was achieved experimentally. It will be noted that the negative slope has been improved through the use of the compensating material. The temperature compensation may be further enhanced by insertion of the low expansion rate material in other reentrant portions of the cavity resonators as will be evident by referring now to FIG. 7.

In this embodiment the dual cavity resonators are designated 74 and 76. The capacitive gaps 78 and 80 are defined by conical electrode me'mbers82, 84, 86, 88. Members 84 and 86 are back-to-back and in this embodiment the entire area between the cavity resonators including the conical members may be fabricated by a body member 90 of the low thermal expansion material including the cone members 84 and 86. The low expansion material also restricts volume increase of the cylindrical outer walls. The remaining conical members as well as the major portion of the cavity resonator walls are fabricated of a high thermal expansion material such as copper. In this embodiment a small amount of copper plating is required on the conical members 84 and 86 as well as the interior walls 92 and 94.

Another illustrative embodiment is shown in FIG. 8 demonstrating the flexibility of the invention. In this view similar structure has been similarly numbered to the view shown in FIG. 7. The body member 90 of the low themial expansion material may now be reduced in diameter to reduce the amount of compensation desired. The main cavity member 95 is enlarged as at 96 to provide for seating of body member 90. The cavity walls 92 and 94 of the low thermal expansion material are slightly reduced in this embodiment.

ln FIG. 9 still another alternative embodiment is shown wherein a conical member 98 fabricated entirely of the low expansion material extends into an axial passage in cavity block 100 defining cavity resonator 102. Opposing electrode 104 defines with member 98 the capacitive gap. The remainder of the passageway within the block 100 is enclosed with a cylinder 106 of a conventional high temperature material.

Many other variations or modifications will readily occur to those skilled in the art. Any and all such alterations are considered to fall within the precepts of the invention. Accordingly, it is understood that the invention is to be limited solely by the spirit and scope of the appended claims.

lclaim:

l. A microwave cavity resonator comprising:

a member of a conductive material having a predetermined thermal coefficient of expansion and defining a hollow cavity of frequency dependent dimensions;

metallic structure-defining reentrant members within said cavity and providing a spaced gap of frequency dependent dimensions;

means of a material having a sufficiently lower thermal coefficient of expansion relative to said conductive member to constrain movement of at least one of said reentrant members with variations in temperature; and said constraining means being disposed outside of said cavi- 2. A microwave cavity resonator comprising:

a member of a conductive material having a predetermined thermal coefiicient of expansion and defining a hollow cavity;

oppositely disposed metallic members spaced within said cavity and defining a frequency-determining gap therebetween; v

thermal-compensating means for constraining movement of at least one of said gap-defining members relative to a predetermined fixed reference plane to thereby cause said gap to widen at an accelerated rate with increasing temperature variations; and

said thermal-compensating means comprising a body of a material having a lower thermal coefficient of expansion relative to said cavity member material associated with at least one of said gap-defining members and disposed outside of said cavity.

3. A microwave cavity resonator comprising:

a conductive member having wall structure of a predetermined thermal coefficient of expansion material defining a hollow cavity having a frequency dependent volume;

said cavity member having a substantially negative thermal coefficient of frequency characteristic with variations in temperature;

oppositely disposed reentrant members within said cavity defining therebetween a spaced gap of frequency dependent dimensions;

means for thermally compensating said gap-defining reentrant members to introduce a thermal coefficient of frequency characteristic which reduces the aforesaid negative characteristic; and

said compensating means comprising a body member of a material having a lower coefficient of expansion than said cavity member material associated with at least one of said reentrant members and disposed outside of said cavity-defining wall structure.

4. A microwave cavity resonator according to claim 3 wherein said thermal-compensating member is embedded within at least one of said reentrant members.

5. A microwave cavity resonator according to claim 2 comprising a plurality of hollow cavity members and back-to-back aligned gap-defining reentrant members within each cavity with intervening metallic supportingwall structure fabricated of a highly conductive metal having said body of thermal compensating material embedded therein. 

1. A microwave cavity resonator comprising: a member of a conductive material having a predetermined thermal coefficient of expansion and defining a hollow cavity of frequency dependent dimensions; metallic structure-defining reentrant members within said cavity and providing a spaced gap of frequency dependent dimensions; means of a material having a sufficiently lower thermal coefficient of expansion relative to said conductive member to constrain movement of at least one of said reentrant members with variations in temperature; and said constraining means being disposed outside of said cavity.
 2. A microwave cavity resonator comprising: a member of a conductive material having a predetermined thermal coefficient of expansion and defining a hollow cavity; oppositely disposed metallic members spaced within said cavity and defining a frequency-determining gap therebetween; thermal-compensating means for constraining movement of at least one of said gap-defining members relative to a predetermined fixed reference plane to thereby cause said gap to widen at an accelerated rate with increasing temperature variations; and said thermal-compensating means comprising a body of a material having a lower thermal coefficient of expansion relative to said cavity member material associated with at least one of said gap-defining members and disposed outside of said cavity.
 3. A microwave cavity resonator comprising: a conductive member having wall structure of a predetermined thermal coefficient of expansion material defining a hollow cavity having a frequency dependent volume; said cavity member having a substantially negative thermal coefficient of frequency characteristic with variations in temperature; oppositely disposed reentrant members within said cavity defining therebetween a spaced gap of frequency dependent dimensions; means for thermally compensating said gap-defining reentrant members to introduce a thermal coefficient of frequency characteristic which reduces the aforesaid negative characteristic; and said compensating means comprising a body member of a material having a lower coefficient of expansion than said cavity member material associated with at least one of said reentrant members and disposed outside of said cavity-defining wall structure.
 4. A microWave cavity resonator according to claim 3 wherein said thermal-compensating member is embedded within at least one of said reentrant members.
 5. A microwave cavity resonator according to claim 2 comprising a plurality of hollow cavity members and back-to-back aligned gap-defining reentrant members within each cavity with intervening metallic supporting wall structure fabricated of a highly conductive metal having said body of thermal compensating material embedded therein. 